Known is a bearing device provided with an anti-rotation mechanism preventing rotation of an outer ring of a bearing. As an example, Patent Literature 1 discloses an anti-rotation mechanism having a long hole reaching the radial middle of an outer ring through a housing supporting the outer ring with a pin inserted in the long hole. Patent Literature 2 and Patent Literature 3 each discloses a bearing device provided with an anti-rotation mechanism inserting a key member in a rotary shaft line direction.